I like Ki Ryn's logic here better than any other. The combat system is an attempt to model actions over time. This use of the ready action is completely contrary to that effort.
The readied action and the triggering action should occur simultaneously (otherwise, how will you force that concentration check?). Consider the following combat sequence:
Initiative count 20
Thuggus: 'I ready my action: If Xyar begins to cast a spell, I charge 10' and attack him'
Initiative count 17
Xyar: 'I cast Magic Missile at Thuggus.'
DM: 'This triggers Thuggus' ready action, which will occur before the triggering action.'
Initiative count 18
Thuggus: 'I charge,' <roll> <roll>, 'and hit for 14 hp damage'
DM: 'Ok... hold on Xyar, this big hulking warrior dashes up and clobbers you with his Warhammer as you are casting your spell.'
Initiative count 17
Xyar: 'Ok... since there's a big hulking fighter in front of me I will avoid the AoO by not casting my spell.'
DM: 'Um... ok in that case you didn't trigger Thuggus' readied action, and he's not standing in front of you. Xyar, give yourself 14 hit points back, and Thuggus move your miniature a couple squares back.'
Thuggus: 'Huh??'